1. Field of the Invention
The present invention relates generally to the field of light emitting diode (LED) devices. More specifically, it relates to an improved electrode structure of LED devices, which are capable of enhancing light extraction.
2. Description of the Prior Art
As known in the art, light emitting diodes (LEDs) are solid-state semiconductor devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and high efficiency for converting electric energy to light.
An LED generally comprises an active layer of semiconductor material sandwiched between p-type and n-type doped layers. A drive current is applied across electrical contacts on the doped layers causing electrons and holes to be injected from the doped layers into the active layer. The electrons and holes then recombine to generate light that emits omnidirectionlly from the active layer and escapes from surfaces of the LED.
When an LED is energized, light emitting from its active layer reaches the emitting surfaces at many different angles. Typical semiconductor materials have a high index of refraction (n≈2.2-3.8) compared to ambient air (n=1.0) or encapsulating epoxy (n≈1.5). According to Snell's law, light traveling from a region having a high index of refraction to a region with a low index of refraction that is within a certain critical angle (relative to the surface normal direction) will cross to the lower index region. Light that reaches the surface beyond the critical angle will not cross but will experience total internal reflection (TIR). The TIR light can continue to be reflected within the LED until it is absorbed, or it can escape out of surfaces other than the emission surface.
Further, the contact electrodes or metal pads disposed on the top surfaces of the doped layers may shield, or in some cases, absorb the light emitted from the active layer. Such internal absorption limits the amount of light that can be extracted from an LED and undesirably reduces the illumination efficiency thereof. Accordingly, there is a need in this industry to provide an improved LED structure that has enhanced brightness and/or efficiency.